In the design of industrial fluid flow systems, it is standard engineering practice to select different types of valves for shutoff purposes than for dynamic fluid control purposes. Fluid shutoff valves are selected for their tight sealing characteristics so as to minimize fluid leakage. On the other hand, fluid control valves are selected for their ability to precisely control dynamic fluid flow characteristics, such as flow rate or pressure, through the fluid flow system. Unfortunately, the requirements for fluid shutoff and dynamic fluid control are often contradictory. Consequently, improvements in one or the other of the sealing and the dynamic fluid control characteristics is usually made only at the expense of the other. Thus, when both precise control and tight shutoff are needed, two separate valves are usually employed. For example, in a system designed for control of a highly corrosive process media, it is common practice to employ a globe valve for throttling purposes and a plug valve for shutoff purposes. These two different types of valves are structurally dissimilar.
Globe valves used for dynamic fluid flow control purposes, such as throttling, are frequently all metal structures which never fully closed in operation. A typical globe valve includes a tapered closure member which is axially moved toward and away from a seating position so as to provide a variable opening in the space between the closure member and the seat. Flow rate and pressure of the controlled process fluid are varied by axial positioning of the closure member with respect to the seat. Although such a valve structure has excellent control capabilities, it does not seal efficiently. Because all of the sealing surfaces are subject to erosion and direct fluid impingement, it is difficult to employ "soft" seat materials, such as plastics, which are necessary to achieve "bubble tight" sealing for long life. Hence, globe valves generally do not use "soft" seats in corrosion fluid applications and do not meet the sealing requirements needed for tight shutoff.
A plug valve, on the other hand, provides excellent shutoff capabilities. A typical plug valve includes a plug member which is rotatably disposed within a fluid flow passage extending through a valve body. The plug member also contains a through fluid passageway which is brought into and out of registry with the flow passage of valve body as the plug member is rotated. The plug member is normally rotated through a 90 degree range of movement between its fully open and fully closed positions. In order to prevent leakage between the valve body and the plug member, a sealing member in the form of a sleeve is fitted in the body about the plug. The sleeve, which is typically formed of a relatively soft, chemically inert plastic material, such as polytetrafluoroetheline, is aperatured in correspondence to the plug member when the plug member is in the fully open position. Significantly, the sleeve, which forms the seat of the plug valve, is protected from direct process fluid impingement by the plug member whenever the valve is in the fully open or fully closed positions.
It is possible to position the plug member intermediate the fully open and closed positions and to use a conventional plug valve for both shutoff and dynamic fluid control purposes. However, such use has serious limitations and disadvantages. First of all, the plug member is in tight fitting relationship with the sealing sleeve. This results in high friction between the plug and sleeve and places high torque requirements on the actuator used to rotate the plug member. It also follows that the plug member rotates relatively slowly and imprecisely.
Perhaps even more importantly, a plug valve is designed to operate only in fully open or fully closed positions where the sleeve is protected against the fluid flow of the process fluid flowing through the valve. When the plug member is rotated to an intermediate position, the sleeve (or seat) is exposed to direct fluid impingement of the process fluid. Such direct fluid impingement leads to erosion and deterioration of the sleeve and results in dimination of the sleeve sealing performance. Although the sleeve also is exposed to process fluid flow in conventional plug valve operation, such exposure occurs only during the brief transitory period of movement between the fully open and fully closed positions. Erosion is obviously much more pronounced during extended exposure, such as in a throttling application. The erosion problem is particularly acute when the process media is a high pressure slurry containing abrasive particles.
One relatively successful attempt at using a plug valve for both shutoff and dynamic fluid control is the caged plug valve. In the caged plug valves of the prior art, a metal cage assembly is located within the plug member and fixed relative to the valve body. This metal cage assembly serves to protect the sleeve from erosion in throttling applications by shielding the soft seat from direct impingement at the valve ports. The cage mechanism also serves to reduce turbulence and the cutting action of high velocity liquids, slurries, and gaseous vapors by providing a more direct, contoured flow path through the valve. While this type of caged plug valve offers substantial advantages over other prior art devices, it is not without its shortcomings. Like other plug valves of the prior art, the above-described caged plug valves has relatively high torque requirements and is relatively imprecise in its positioning capabilities. Furthermore, such caged plug valves do subject the seat to turbulent process media flow and process media cavitatation. This, of course, leads to seat erosion and spoils the tight shutoff sealing capabilities of the valve.